1. Technical Field
The technical field is registration of features in innerlayer printed wiring board panels. More particularly, the technical field includes X-ray alignment of embedded features to their electrical terminations and to other associated circuitry.
2. Background Art
The practice of embedding passive circuit elements and other features in printed wiring boards (PWB) allows for reduced circuit size, improved circuit performance and for additional semiconductors to be placed on the board surface. Features such as passive circuit elements and other components are typically embedded in panels that are stacked and connected by interconnection circuitry, the stack of panels forming the printed wiring board. The panels can be generally referred to as “innerlayer panels.”
Passive circuit elements can be fabricated by a number of methods. “Formed-on-foil” elements such as resistors, for example, are formed by selectively depositing a thick-film resistor material on a metallic foil. Capacitors are formed on foil by depositing a thick-film dielectric and conductor combination. Such elements can be fired under thick-film firing conditions or cured at low temperatures. The resultant passive elements are then laminated to a dielectric and to a second foil, forming an innerlayer panel. The foils are disposed on the exterior of the innerlayer panel, and are used to form circuitry.
A printed wiring board is formed by photo-etching the foils on the exterior of the innerlayer panel to create circuitry, and then stacking and laminating the panels. The circuitry must be accurately positioned on the innerlayer panel in order to properly interconnect circuitry of the stacked panels in the printed wiring board. The circuitry is formed by punching register holes at specific locations in each innerlayer panel, and using register pins to position a photo-tool. The photo-tool is used to image circuitry on the exterior of the panels. The circuitry of the innerlayer panels is thereby correctly aligned during lamination.
Embedded circuitry, components and other features in an innerlayer panel must also be accurately interconnected to the circuitry on the exterior of the innerlayer panel. The alignment process is difficult, however, because embedded features are not visible. One alignment process involves the use of fiducials as position locators in an innerlayer panel. The fiducials are printed on a foil at the same time as embedded features during formation of the panel. Regions of the foil containing the fiducials are selectively etched away in the vicinity of the fiducials in order to expose the fiducials. Register holes are then punched in the innerlayer panel in known positions relative to the now visible fiducials. The register holes are used to position a photo-tool for further etching of the foil, which results in circuitry on the exterior of the panel.
Etching away the foil to expose fiducials allows for alignment of embedded features with other circuitry of the innerlayer panel. The etching processes, however, add time and cost to the alignment process.